The invention relates to modifying the bit rate of a data stream having the following characteristics. The data stream comprises control words. A control word defines when a data portion comprised in the data stream has to be read from an input buffer at a receiving end. A video data stream in accordance with a standard of the Moving Pictures Experts Group (MPEG), has such characteristics.
U.S. Pat. No. 5,680,483 describes an MPEG encoder with switchable bit rates.
It is an object of the invention to allow a better quality of the data comprised in the data stream.
The invention takes the following aspects into consideration. Let it be assumed that, at a certain instant, it is established that the bit rate of a data stream should be increased in order to maintain a certain level of quality. Ideally, the bit rate is modified instantly. That is, ideally, an immediate response is given to a demand for bits. However, for a data stream having the characteristics mentioned hereinbefore, the bit rate can not immediately be modified. If the bit rate were immediately modified, this would probably cause underflow or overflow of the input buffer at the receiving end. In that case, data would be lost which is detrimental to quality.
The background art seems to apply the following principle. The bit rate is modified only after every control word which has already been defined, has taken effect at the receiving end. In MPEG terminology this means that the bit rate is modified with a delay corresponding to the end-to-end delay. Accordingly, it is ensured that neither underflow nor overflow will occur whatever the bit-rate modification is.
According to the invention, it is calculated, for at least one future instant in time, within which bounds the bit rate of the data stream can be modified at that future instant in time so as to cause neither underflow nor overflow of the input buffer. The rate is modified at a future instant in time, within the bounds which have been determined for the future instant in time.
Accordingly, the invention allows to modify the bit rate within a relatively short delay while preventing underflow and overflow. It may happen the bit rate can not be switched, as it were, to a certain desired bit rate within a relatively short delay. However, the bit rate can be switched to an approximation of the desired bit rate, the approximation being one of the aforementioned bounds. The latter will generally provide a better response to a demand for bits, than waiting for a relatively long fixed delay in order to apply the desired bit rate such as in the background art. Consequently, the invention allows a better quality of the data comprised in the data stream.
The invention and additional features, which may be optionally used to implement the invention to advantage, are apparent from and will be elucidated with reference to the drawings described hereinafter.